The present invention relates to a method for operating an exhaust gas after-treatment system for an internal combustion engine.
Internal combustion engines are known which can be operated with a gas containing methane, for example natural gas or methane as well as with a mixture of gas and another fuel, for example diesel fuel (dual fuel).
Pure gas engines are often derived from gasoline or diesel engines, wherein an externally supplied ignition is generally used to ignite the gas/air mixture with the aid of spark plugs. In the case of diesel/gas engines, the engine basically relates to a diesel engine which allows for a pure diesel operation as well as for a mixed operation consisting of diesel fuel and gas. In this case, a portion of the diesel calorific value is replaced by gas. The ignition of the total fuel, i.e. the diesel-gas/air mixture occurs via the diesel portion. Substitution rates of the diesel fuel by gas are hereby possible up to 70%.
In all approaches which are at least partially based on the combustion of gas containing methane, there is the problem of high, untreated methane emissions. Primarily for reasons of environmental protection, the methane emissions have to be reduced within the scope of an exhaust gas after-treatment. For this purpose, methane oxidative coupling catalysts (MOC) are known which oxidize the methane contained in the exhaust gas on the basis of palladium rich formulations. To this end, formulations can be used which have a weight ratio of palladium (Pd) to platinum (Pt) of, for example, up to 7:1 and even greater. Other methane oxidative coupling catalysts are based on palladium-only formulations, such as, for example, Pd/aluminum oxide. A certain methane conversion can, however, generally first be observed in the case of such formulations above 400° C. Often temperatures well over 500° C. are required for the complete oxidation of the methane. Such temperatures are however only seldom achieved in the energy-efficient, gas-engine lean operation.
In order to reduce the nitrogen oxide emissions of an internal combustion engine, it is known to install an SCR catalyst in the exhaust gas region of the internal combustion engine. In an SCR catalyst, the nitrogen oxides (NOX) contained in the exhaust gas of the internal combustion engine are reduced to nitrogen in the presence of a reducing agent. Ammonia (NH3), which is added to the exhaust gas, is required for the reactions to take place. NH3 or reagents that split off NH3 are used as the reactive agent, for example an aqueous urea solution which is injected into the exhaust gas tract.
Approaches already exist for using non-thermal plasmas for an exhaust gas after-treatment. For example, the translation of the European patent publication DE 603 23 090 T2 describes a reactor for the plasma treatment of exhaust gases of an internal combustion engine. In this technology, which is based on non-thermal plasmas, metastable species, free radicals and highly reactive ions are formed, which can be used for treating exhaust gases, by the collision of gas molecules with very energy rich electrons, which are generated by means of an electrical discharge.